To move sideways in a furtive or sneaky way, often to approach something without being noticed.
From the noun 'side,' with the verb form emerging in the 16th century. The sideways motion mirrors how crabs or suspicious people move—not forward directly, but at an angle.
You can almost see someone sidling in your mind—shoulders first, eyes averted, moving diagonally like they're apologizing with their whole body. It's one of the most physically descriptive verbs in English!
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