10% monthly interest
Slang from the consumer finance industry; 月一 (つきいち) refers to a monthly interest rate of 10%. Not used in general conversation.
I go to see a movie about once a month.
With loan sharks, they sometimes charge 10% monthly interest.
Standard kanji spelling; the numeral 一 is often used in casual writing.
Variant with half-width numeral; common in informal digital text.
Abbreviation of 月に一回 (つきにいっかい, 'once a month'), formed by taking the first character of 月 (つき) and 一 (いち). The pattern is productive for other time units (e.g., 週一, 年一).