Against the grain
Against one's inclination or natural tendency.
The phrase brings to mind the image of the grain in wood, which, if planed in the wrong direction, will tear rather than lie smoothly. That may not have been in the mind of whoever coined the phrase, as none of the early citations of the phrase refer to wood.
It was used by Shakespeare, in Coriolanus, 1607:
Say, you chose him
More after our commandment than as guided
By your own true affections, and that your minds,
Preoccupied with what you rather must do
Than what you should, made you against the grain
To voice him consul: lay the fault on us.
... and then by Thomas Hubbert, in A pill to purge formality, 1650:
"O this goes against the grain, this cannot be indured."
Grain is recorded as meaning 'tendency, nature, inclination', but not until after Shakespeare's use of it above. It may be that he was alluding to the grain in timber - hard to say.
See other phrases and sayings from Shakespeare.