Flat adverb

A flat adverb is an adverb that assumes the form of a related adjective, most often when words ending in -ly are used without the -ly. Though once quite common, flat adverbs have been largely phased out by their -ly counterparts. This shift owes to 18th-century grammarians who insisted that adverbs end in -ly. Nonetheless, flat adverbs are preferred in some idioms, as in "take it easy" and "sleep tight".