[MMM]

I would just add that although 約束 does mean "commitment" it can also mean other things in English. Personally I think it would look silly as a kanji tattoo, as it is a word that can also mean "Let's meet for lunch next week. Ok, 'promise'" and doesn't necessarily imply a lifelong commitment.

I think if I saw that on your finger I would assume you lost a bet where the loser had to "promise" to get a kanji tattoo. And by getting it you were holding up your end of the bargain...but that's just me.

約束 【やくそく】 (n,vs) (1) arrangement; promise; appointment; pact; engagement; (2) convention; rule;