Grain is a word like corn. Not only do the two words have very similar meanings and uses, but they are both words whose usage is rather different in British English and American English.
The basic meaning of grain, like 'corn', is 'the fruit, or seed, of cereal crops - the grass-like plants grown for food'. This became the collective noun for the crop itself. It is used in different areas for different types of cereal crop. This is where the transatlantic difference in usage occurs: "In England the colloquial word for this sense is corn, which in the U.S. has a different application" (OED) The American usage is mostly for maize - see further corn.
Other meanings have developed for the word 'grain' that may be of interest to you.
- There is a general sense of 'a small unit', drawn from the individual seed. If you say "There is a grain of truth in what he says", you mean that there is a small element of solid fact, but you are implying that a great deal of what he says is not true.
- This gave rise to an old unit of weight, the smallest in English systems. The grain "is a unit of mass now equal to exactly 64.79891 milligrams, in all English mass and weight systems (avoirdupois, Apothecaries' and troy). An avoirdupois ounce is equal to 437.5 grains, whereas a troy ounce is equal to 480 grains. With the differing number of ounces in their respective pounds, an avoirdupois pound is 7,000 grains, whereas a troy pound is 5,760 grains. There have been other English pounds with round sizes in grains." (Wikipedia).
- It has some figurative uses, as when 'grains of Paradise' was the name for the seeds of a variety of the spice cardamom.
- The term grain is also applied to surfaces composed of apparent units, or particles. In rock, the particles may be immediately visible: one can usually see that sandstone is composed of grains of sand. In leather, the grain is akin to the pores in the skin of the animal. In photography, the grain is the closeness of the particles of light-sensitive chemicals which coat the paper, and thus of the quality, or sensitivity to light, of the paper.
- In wood, on the other hand, and some other substances, the grain is the lengthwise arrangement of the cells which makes it clear to the craftsman how the material should be cut. In wood, this leads to pleasing patterned effects which may be used as a decorative embellishment to the material, in furniture for example.
- So 'to go against the grain' is to oppose a natural tendency. Wood is easy to cut, and naturally splits, with the grain; so if a suggestion 'goes against your grain', it will be against your natural tendency.