Learn English – Origin of the word “delete”

I don't think that it's the same prefix as much as it is the remnant of a number of different grammaticalised pre-fixings. Most of them seem to have happened during the period when then curious admixture of French, Viking Danish, Anglo-Saxon and a sprinkling of Gaelic were distilling themselves down into the various dialects of Middle English. The spelling's kind of arbitrary, but a is the letter we tend to use when a word starts with a schwa that's flatter than we'd represent with a short e.

English was essentially an unwritten language during that period (and the population essentially illiterate), so it could be a time of great flux. A lot of words were repartitioned. All one became alone, the n sound migrated from the ends of words like mine and thine to become the initial (and previously non-existent) consonant of words (especially eke names -- thine eke name also became thy nickname) and so on.

As with a lot of what happened during this period, what we have now in the language is mostly what was present in and around London when the orthography was fixed by printing. Many of the a- words that one recognises as quaint regionalisms today (like a-hunting) were standard in dialects that did not, themselves, have the good fortune to become the standard themselves.

As for new words, well, printing (and general literacy) sort of put a stop to arbitrary movement of word boundaries. A question was posted here earlier asking about the meaning of "grab a hold", and it didn't take long for somebody to reply that the phrase was actually "grab ahold" (something my spell checker has decided is a problem). I would bet that "grab a hold" or "take a hold" (hold being synonymous with handle, as preserved in hand-hold) was the original phrase. They sound the same, and if you hear a hold often enough without seeing it written down, there's no real reason why you might not think of them as a single word.

The etymonline entry for assassin is in agreement with the OED entry for it, and in agreement (as far as it goes) with several other online sources (1,2,3,4,5). The last of those links (from thefreedictionary.com) provides some additional information (highlighted in following) that is not mentioned in etymonline or OED:

Word History: Active in Persia and Syria from the 8th to 14th centuries, the original Assassins were members of the Nizaris, a Muslim group who opposed the Abbasid caliphate with threats of sudden assassination by their secret agents. Other populations of the area regarded the Nizaris as unorthodox outcasts, and from this attitude came one of the names for the group [حشاشين (ḥaššāšīn, “hashish users”), or أساسيون (ʾasāsiyyūn)] a word originally meaning "hashish users," which had become a general term of abuse. Reliable sources offer no evidence of hashish use by Nizari agents, but sensationalistic stories of murderous, drug-crazed [ʾasāsiyyūn] or Assassins were widely repeated in Europe. Marco Polo tells a tale of how young Assassins were given a potion and made to yearn for paradise – their reward for dying in action – by being given a life of pleasure. As the legends spread, the word [ʾasāsiyyūn] passed through French or Italian and appeared in English as assassin in the 16th century, already with meanings like "treacherous killer."

If the above is correct, because Marco Polo was Italian, the story he brought back would have spread in Italy first, and from there to France, Spain, England, etc.

A problem with that explanation is that OED shows the Latin form Assisinos in use already in 1237, which predates Marco Polo's birth by around 17 years. The first Crusades began in 1095, around 160 years before Polo.