爪に火を点す
(Tsume ni hi wo tomosu; “To light one’s (finger)nail”)

Definition:

Extreme penny-pinching. To be incredibly poor, or just incredibly stingy with one’s money. Being unable to afford even a candle – or unwilling to buy one – and instead making light at night by setting flame to one’s own nails.

Breakdown:

This is grammatically a complete sentence, although you’ll usually see it as a clause or phrase in a longer sentence. The verb is 点す (alternately 灯す), (tomosu), “to light (a fire).” The particle を (wo) marks as direct object of this verb the preceding noun 火 (hi, rhyming with English pronoun “he”), “fire.” Finally we have the particle に (ni) in its function as a location-marker, acting on the noun 爪 (tsume), “finger/toe nail.”

Notes:

It’s not uncommon to render tomosu in kana as ともす, and 灯す is a second valid kanji to use, but replacing tomosu with 燃やす (moyasu, “to burn”) is an error.

This is the つ entry for the Osaka iroha karuta set. One of my sources also ties it to Kefukigusa, an early Edo-period collection of haiku compiled by Matsue Shigeyori.

Example sentence:

「大学生と大学院生の時は、爪に火を点すほど貧しい暮らしをして来たので、外食ですらなんとも贅沢な気持ですよ」

(“Daigakusei to daigakuinsei no toki wa, tsume ni hi wo tomosu hodo mazushii kurashi wo shite kita no de, gaishoku de sura nan to mo zeitaku na kimochi desu yo.”)

[“As an undergrad and grad student, I was poor enough to burn my nails for light, so simply eating at a restaurant feels ”]