"slantingly, in a sloping direction," 1826, from slant (n.) + way (n.) + adverbial genitive -s. Slantwise is attested from 1570s (see wise (n.)). Slantly (1727) is perhaps only in poems and dictionaries.
Entries linking to slantways
slant n.
1650s, "an oblique direction or plane" (originally of landforms), from slant (v.) or its adjective. The meaning "way of regarding something, a mental bias" is from 1905. Derogatory slang sense of "slant-eyed Asian person" is recorded from 1943, from earlier slant-eyes (1929).
way n.
Old English weg "road, path; course of travel; room, space, freedom of movement;" also, figuratively, "course of life" especially, in plural, "habits of life" as regards moral, ethical, or spiritual choices, from Proto-Germanic *wega- "course of travel, way" (source also of Old Saxon, Dutch weg, Old Norse vegr, Old Frisian wei, Old High German weg, German Weg, Gothic wigs "way"), from PIE root *wegh- "to go, move, transport in a vehicle."
From c. 1300 as "manner in which something occurs." Adverbial constructions attested since Middle English include this way "in this direction," that way "in that direction," both from late 15c.; out of the way "remote" (c. 1300). In the way "so placed as to impede" is from 1560s.
From the "course of life" sense comes way of life (c. 1600), get (or have) one's way (1590s), have it (one's) way (1709). From the "course of travel" sense comes the figurative go separate ways (1837); one way or (the) other (1550s); have it both ways (1847); and the figurative sense of come a long way (1922).
Adverbial phrase all the way "completely, to conclusion" is by 1915; sexual sense implied by 1924. Make way is from c. 1200. Ways and means "resources at a person's disposal" is attested from early 15c. (with mean (n.)). Way out "means of exit" is from 1926. Encouragement phrase way to go is short for that's the way to go.
wise n.
"way of proceeding, manner," Old English wise "way, fashion, custom, habit, manner; condition, state, circumstance," from Proto-Germanic *wison "appearance, form, manner" (see wise (adj.)). Compare Old Saxon wisa, Old Frisian wis, Danish vis, Middle Dutch wise, Dutch wijs, Old High German wisa, German Weise "way, manner." Most common in English now as a word-forming element (as in likewise, clockwise); the adverbial -wise has been used thus since Old English. For sense evolution from "to see" to "way of proceeding," compare cognate Greek eidos "form, shape, kind," also "course of action." Ground sense is "to see/know the way."