Slantwise means at a slope rather than straight. Rain coming slantwise.
The -wise ending means in the manner of, as in lengthwise and clockwise. Slantwise is rare and mostly literary, and at an angle is what people usually say.
1adverbslantwiseAt a slope rather than straight.
“The light came slantwise through the shutters all afternoon.”
slantwise is a manner.
Proto-Germanic
*slintaną
English
slant
English, today
slantwise
From slant + -wise. slant Late Middle English -, from a variant of the earlier form dialectal slent, from Old Norse - or another gmq - source, cognate with Old Norse slent, Swedish slinta (“to slip”), Norwegian slenta (“to fall on the side”), from Proto-Germanic slintaną. Probably influenced by aslant.
Etymology from Wiktionary, CC BY-SA
Practised in A Song of Ice and Fire