(1 syl.). To strain courtesy. To stand upon ceremony. Here, strain is to stretch, as parchment is strained on a drum-head. When strain means to filter, the idea is pressing or squeezing through a canvas or woollen bag.
Strain at a gnat and swallow a camel. To make much fuss about little pecadillos, but commit offences of real magnitude. “Strain at” is strain out or off (Greek, di-ulizo). The allusion is to the practice of filtering wine for fear of swallowing an insect, which was “unclean.” Tyndale has “strain out” in his version. Our expression “strain at” is a corruption of strain-ut, “ut” being the Saxon form of out, retained in the words ut-most, utter, uttermost, etc.
The quality of mercy is not strained (Merchant of Venice, iv. 1)- constrained or forced, but cometh down freely as the rain, which is God's gift.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894