means Having a sharp, acidic taste like vinegar or lemon, or by extension turning unpleasant, bitter, or spoiled in mood, milk, or relationships.

from From Old English 'sūr,' meaning acid or tart, with deep Germanic roots — cousins survive in German 'sauer' (think Sauerkraut, literally 'sour cabbage') and Dutch 'zuur.' The whole family likely traces back to a Proto-Indo-European root suggesting something acid or moist. The taste sense came first; the metaphorical 'sour mood' and 'gone sour' senses are later sourings of the same word.