To act obsequiously toward someone in authority; to behave like a servile flatterer or yes-man in hopes of gaining favor.
From 'boot' (footwear) plus 'lick' (to lap with tongue); the image is of someone so submissive they would lick boots, conveying ultimate servility and humiliation.
The power of 'bootlick' comes from its gross, vivid imagery—it's so repulsive that a single word does the work of describing pages of flattering behavior, and it's been used in this sense since the 1600s to shame people who grovel before authority.
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