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.
ノミ行為は法律で禁止されている。
Bookmaking is prohibited by law.
その証券会社はノミ行為で摘発された。
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.”