E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Strain (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 peccadillos, 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 Gods gift.