I suspect that the Jisho.org entry is reflecting sense ③ in the Nihon Kokugo Daijiten (NKD) entry visible here at Kotobank. Specifically:
③ (生【い】きるよりどころの意味【いみ】から特殊化【とくしゅか】して) 一生【いっしょう】をそれに捧【ささ】げてもよい誠意【せいい】を示【しめ】す証拠立【しょうこだ】ての文字【もじ】、また、転【てん】じてその語【ご】。多【おお】く遊里【ゆうり】に行【おこ】なわれた習慣【しゅうかん】で、相愛【そうあい】の男女【だんじょ】が互【たが】いに二【に】の腕【うで】へ「命【いのち】」の一字【いちじ】、または「誰々【だれだれ】命【いのち】」と[入]{い}れ墨【ずみ】して、二世【にせ】も三世【さんぜ】もと誓【ちか】った。
(3) (specialized from the meaning of one's reason for living) character serving as proof showing that one is in earnest in giving up one's life for something, or, by extension, such a word. Common custom in the red-light districts [of the Edo period], when a man and woman in love with each other would have the [命]{inochi} ["life"] kanji tattooed on their upper arms, or "[NAME] [命]{inochi}" ["NAME life"], and pledge even to their second or third incarnations.
The Daijisen entry on that same page says much the same thing as the NKD, as does my local copy of Daijirin.
The quote given in the NKD dictionary entry as cited evidence of this usage is dated to 1678. That, plus the mention of the 遊里【ゆうり】 or Edo-period red-light district, leads me to believe that this is probably an historical custom that is no longer in common practice (if at all).