means The act of refusing or turning down a person, thing, or idea — the formal 'no' to an application, an offer, a person, or a transplanted organ.

from From Latin 'reicere,' to throw back — 're-' (back) plus 'iacere' (to throw, the same root that hurls itself through 'eject,' 'project,' and 'inject'). The past participle 'reiectus' became Old French 'rejeccion' before settling into English. So at its heart, rejection is exactly what it feels like: something thrown back at you.