From the mochikomi page:
Mochikomi is an invasion which is a total failure - no redeeming features. It is a very amateurish thing to happen to one's stones, since such an invasion is almost certainly aji keshi, and probably loses points too.
Chris Hayashida: What is the etymology of this word? I have a visual image of New Year's rice cakes being tossed about. I got this from "komi," which I understand to mean "(something) thrown in" as in "the points thrown in at the end" but I'm sure it's a different verb.
持ち込む - (mochikomu) the komi's right, but the mochi comes from motu, to carry, unless i'm much mistaken, kokiri
Bill: I think that the relevant sense of motsu is have or take. Komu is an intensifier. A stone which is mochikomi is not just taken, it's really taken, i. e., in the context of go, taken for free (or without adequate compensation).
Bob Myers: When this word is not written in katakana, it's written with the characters shown above. However, I've rarely seen the verb form mochikomu--it's (nearly) always the nominal form mochikomi. In other words, it seems like mochikomi is not something you do, it's a thing done to you, or a place you end up. mochikomi ni naru, mochikomi ni sareta.
However, I cannot put together the meanings of motsu and komu in any way that relates to this concept of failed invasion. (motsu does not mean take, certainly not in the go sense anyway.) So I'd like to present an off-beat etymology which is that mochikomi is a sort of pun, taking the word uchikomi (invasion), and to indicate its failed nature, replacing the uchi part with the mochi of shirimochi meaning to fall on your rear-end. (And yes, it turns out that this mochi *is* the glutinous rice cake Chris mentioned.) Take that, eminent Japanese scholars!
Fhayashi: The feeling for mochikomi is someone bringing you a gift. Like 'coming to visit bearing gifts'. There's no pun association with any go term.