"to move or go sideways," 1690s, back-formation from obsolete Middle English sidlyng (adv.) "obliquely, sideways; aslant; laterally" (early 14c., perhaps in Old English), from side (n.) + adverbial suffix -ling; altered on analogy of verbs ending in -le. Related: Sidled; sidling. Old English had sidlingweg (n.) "sidelong-way, oblique road."
