also: のみこうい
bookmaking; bucketing
Illegal gambling practice where a bookmaker or broker takes the opposite side of a customer's bet or order without actually executing it on an exchange. ノミ is a metaphor from a flea sucking blood, implying the operator 'sucks' the customer's money.
Bookmaking is prohibited by law.
That brokerage was busted for bucketing.
Standard spelling using katakana ノミ for the 'flea' metaphor and kanji 行為 for 'act'.
Ateji-like kanji spelling using 呑み (drink/swallow) for the phonetic value; rarely used.
ノミ represents 呑み, from 呑む in the jargon sense of accepting a bet or securities order without passing it on or executing it; 行為 means “act.”