A contraction of Old English 'gea sie' (yea, may it be so) — originally only for contradicting negative questions.
Used to give an affirmative response; expressing agreement, consent, or confirmation.
From Old English 'gēse' or 'gīse,' probably a contraction of 'gēa' (yea, yes) + 'sīe' (may it be so — subjunctive of 'bēon,' to be). Literally 'yea, may it be so.' The 'gēa/yea' element traces to Proto-Germanic *ja (yes, indeed), from PIE *yē (emphatic particle). English is unusual among major European languages in having two affirmation words: 'yea' (affirming a positive statement
Old English had a four-word answer system: 'gēa' (yea) affirmed a positive question, 'nay' denied a positive question, 'yes' contradicted a negative question, and 'no' confirmed a negative one. If someone asked 'Is he not coming?' the correct response was 'yes' (on the contrary, he is) or 'no' (correct, he is not). French