Learn English – Origin of the similarity of “god” and “good”

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.

Ah, I see I should have read more closely.
I actually did some research that touches on this.

The ST- assonance (initial consonant cluster) has a long and varied and surprisingly coherent phonosemantic history, dating from a pre-Proto-Indo-European era.

I tripped over ST- while trying to figure out what style meant, in a paper on metaphors, for a literary journal named Style. As usual, there was a lot more stuff going on than I had expected.

The paper is called "Style Stands Still", and I see no reason to reproduce it here, since it's online. (The stuff about ST- starts on page 8, but it's built on the context of the first part; the paper is 20 pages long, with bibliography).

EDIT: By request, a brief excerpt from the paper:

Figure 1. st-initial PIE roots, with some reflexes in Modern English
Source: Watkins (2000), Pokorny (1959)

• *stā- ‘To stand, with derivatives meaning “place or thing that is standing”’ (Pok sta- 1004)
  style, stand, steed, stud, stay, stage, stamen, standard, stem, station, stasis, static, status, stable, stoic, store, stylite, steer
• *steigh- ‘To stride, step, rise’ (Pok steigh- 1017)
  stile, stirrup, stickle, distich, acrostic
• *steu- ‘To push, stick, knock, beat’ (Pok 2. steu- 1025)
  stub, steeple, stoop, stutter, stock, stoke, steep
• *stel- ‘To put, stand; with derivatives referring to a standing object or place’ (Pok 3. stel- 1019)
  stolon, stalk, stele, stilt, pedestal, stolid, stall, stout
• *ster- ‘Stiff’ (Pok 5. ster- 1029)
  stare, starch, stork, starve, stark, stern, strut, start, stark, startle
• *stebh- ‘Post, stem; to support, place firmly on, fasten’ (Pok steb(h)- 1011)
  stoop, staff, staple, stump, stamp, stomp, stave
• *steip- ‘To stick, compress’ (Pok steib(h)- 1015)
  stubble, stiff, stipple
• *steg- ‘Pole, stick’ (Pok 2. (s)teg- 1014)
  stake, stack, stagger
• *stegh- ‘To stick, prick; pointed’ (Pok stegh- 1014)
  stair, stick, sting, stigma, stimulate, stag

There is a strong family resemblance among the roots here. Indeed, on perusing this list, one finds a persistent cognitive image building up, with at least the following four significant perceptual properties:

Figure 2. Cognitive semantic properties of st-initial PIE roots

1. One-Dimensional: The image has only one salient major dimension
2. Vertical: That dimension is situated in an up-and-down orientation
3. Strong: The image displays rigidity, stability, and physical integrity
4. Still: The image is either unmoving, or frozen in motion

...

Not all of the characteristics in Figure 2 will be true of every Modern English word that comes from the roots in Figure 1, but some combination of the features applies to them all. For instance, although the Modern English words stamen, stile, steeple, stalk, stork, staff, stake, and stick each come from a different PIE root, all of them refer either to long rigid objects or to things characterized by such, mostly vertically oriented, and often supporting, attached, or applied to other structures by their ends. For that matter, they tend to apply to any Modern English word beginning with st-, no matter where it comes from.