Learn English – the 1896 source for the origin of “dyke”

This is a rime; that is, it's the vowel nucleus plus coda of a monosyllable. And many rimes have phonosemantic coherence. As do assonances, that is, initial consonant clusters.

This particular one has a 3-Dimensional sense — an "ump" is something with three dimensions, roughly the same size in all three. There are 15 such simplex words in English:

lump clump dump plump hump slump jump rump
bump crumple stump rumple sump tump mump

Some of the assonances in these words are also phonosemantically coherent. KL- means 'Together', PL- means '2-Dimensional Thick', and ST- means '1-Dimensional Vertical Rigid'. So plump means an ump with wide padding, clump means an ump formed by putting things together, and — my favorite — stump means an ump that used to be a long vertical rigid thing.

See here for much, much more about phonosemantics.

The two earliest instances of preplan/preplanned/preplanning (with or without a hyphen) that a Google Books search finds are from the pen of Robert Southey, who was Poet Laureate of England for the last thirty years of his life (from 1813 to 1843). The first instance is from a letter by Robert Southey to the Reverend Neville White, dated February 19, 1824, reprinted in Selections from the Letters of Robert Southey (1856):

My dear Neville,

Here I am, once more at my desk, by my own fire-side. My movements were all punctually performed, as they had been pre-planned. I reached home on Sunday morning, without impediment or mishap of any kind, and, thank God, found all well. Some little time is required before I can fairly get into joint again, after so complete a dislocation ; and I bring back with me a formidable accumulation of letters, which followed and found me whithersoever I went, and which it was not possible for me to answer during so hurried a mode of life.

The next is a quotation from Robert Southey's "Colloquies on the Progress and Prospects of Society," in a review of that work in Blackwood's Edinburgh Magazine (October 1829):

Montesinos [a character in Southey's dialogue, speaking to Thomas More]. Like the whole fabric of our society, it [the manufacturing system in contemporaneous England] has been the growth of circumstances, not a system pre-planned, foreseen, and diliberately chosen. Such as it is, we have inherited it, or rather have fallen into it, and must get out of it as well as we can. We must do our best to remove its evils, and to mitigate them while they last, and to modify and reduce it till only so much remains as is indispensable for the general good.

From "Shakespeare in Modern Thought," in North American Review (October 1857):

The events and characters of a drama may be very amusing and interesting, may be morally and spiritually edifying ; but unless they proceed from some common point of view, some pervading, centralizing principle, and so are related essentially to one another, having a common life, they do not belong to the domain of rt. There is undoubtedly, such a universal "central idea" for each living man. The dramatic poet, like a presiding deity, surveys all the details, and holds in his hands the threads of connection and relation, so that his representation is no patchwork of circumstances and capricious succession of words and deeds, but a regular and pre-planned figure, woven out of many different and variously colored threads, each of which has its place in the finished product, and is essential to a complete embodiment of the ideal pattern.

From "Long Vacation," in London Society (January 1868):

Then came preparation, packing, and departure. One by one the crew broke away ; cordially we shook hands, and pre-planned réunions in town, at Lord's, in the Highlands, and elsewhere.

From John Scott Russell, Systematic Technical Education for the English People (1869) [the same year as the New York Court of Appeals Decision cited in bib's answer]:

But the growing youth, who has everything to learn, and no help, how is he to find fit education and training for his work in life? For the ordinary English lad, education must be pre-planned, prepared, brought home to him, to his father's home, to his master's workshop. He cannot seek education. We must seek him.

The origin of the word preplan thus appears to rest not with an anonymous twentieth-century purveyor of business jargon, but with a nineteenth-century poet and litterateur; and other early instances of the phrase show its being taken up in the fields of literary criticism, memoir, pedagogy, and law. The modern notion of convening a preplanning meeting before the planning meeting to work out what will be covered and (perhaps) decided at the latter may be a creature of corporate or government bureaucracy; but preplanning itself evidently was not born in a conference room.

With a confirmed first occurrence Google Books search results of 1824, preplan actually beats the earliest confirmed match for the phrase "plan ahead"—which has its own problem with face-value redundancy, and which has a confirmed first occurrence of 1848 in Google Books search results. I see very little change in the sense of preplanned from its meaning as used by Southey in the 1820s and its meaning as given by Oxford and Collins and quoted in Mari-Lou A's question.