To make something illegitimate, impure, or inferior; to corrupt or debase something of value (British spelling).
From bastard + -ise (British suffix forming verbs). The suffix comes from Latin -isare. The verb extends the noun's meaning metaphorically to describe corrupting or degrading things beyond just people's legitimacy.
Interestingly, this metaphorical use shows how humans turn social categories into moral judgments—we went from 'not born in wedlock' to 'corrupted or impure,' revealing medieval attitudes about legitimacy and purity.
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