means A kitchen tool, usually a perforated or mesh bowl, that separates liquids from solids by letting the liquid drain through while holding the food back.

from From the verb "strain," which came through Old French "estraindre" from Latin "stringere," meaning to draw tight, bind, or press — the same root behind "stringent" and "strict." The sense slid from squeezing and pressing to forcing liquid through a barrier, and the "-er" simply names the thing that does the straining.