In a threatening, menacing, or evil manner; with a look or tone suggesting harm or malice.
From 'baleful' (from Old English 'bealh,' meaning evil or dire) combined with the adverbial suffix '-ly,' creating an adverb form describing manner or attitude.
The word 'baleful' comes from the same root as 'bale' (evil)—so when you look at someone 'balefully,' you're etymologically giving them an ancient curse-word stare!
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