Coarse means rough in texture, or rough in manners. Coarse sand, coarse cloth, coarse language.
Of a material it means made of large particles or thick threads: coarse salt, coarse sandpaper, coarsely chopped. Of behaviour it means crude — coarse jokes, coarse language — and it is a mild but real disapproval. Note the homophone trap with course; and coarse fishing, which in Britain means angling for anything except salmon and trout, a distinction with a lot of class history in it.
1adjectivecoarseRough in texture, made of large particles.
“The beach was covered in coarse sand that hurt our bare feet.”
2adjectivecrudeCrude in manner or language.
“His coarse jokes embarrassed everyone at the wedding dinner.”
coarse is a quality of roughness.
It began as the adjective use of course, meaning ordinary or following the usual course, and the spellings split only in the eighteenth century. Ordinary became common became unrefined.
Adjectival use of course that diverged in spelling in the 18th century. The sense developed from '(following) the usual course' (cf. of course) to 'ordinary, common' to 'lacking refinement', with 'not fine, granular' arising from its application to cloth. Compare the development of mean.
Etymology from Wiktionary, CC BY-SA
Practised in Sharper Adjectives, Texture & Touch