Full of danger; hazardous or perilous, though 'dangerous' is the standard modern form of this word.
From 'danger' plus the suffix '-ful' (meaning full of), creating a parallel form to 'dangerous,' though the latter comes directly from Old French and became the standard adjective.
English sometimes creates the same word multiple ways—'dangerful' and 'dangerous' both mean the same thing, but 'dangerous' won because it came from French and felt more prestigious during Middle English standardization.
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