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.
Specifically refers to illegal private horse race betting tickets; ノミ行為 is a broader term covering various forms of illegal bookmaking and 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.”