obscure

First recorded in 1350–1400; Middle English, from Old French oscur, obscur, from Latin obscūrus “dark”

1. See mysterious. 8. See dark.

The adjective obscure first appears in English about 1425 (if not earlier); the verb appears around the same time. The adjective obscure comes from Anglo-French and Middle French oscur, obscur “without light, dark (in color), hard to understand,” from Latin obscūrus “dim, dark, dingy, faint,” an adjective made up of the prefix ob- “toward, against” and the adjective scūrus, which does not occur in Latin.
The verb obscure may simply derive from the English adjective by functional shift (a change in the grammatical function of a word). Alternatively, the verb may derive from Middle French obscurer “to make or become dark” or from Latin obscūrāre “to cover, obscure, overshadow, conceal,” a verb derived from obscūrus.
The unrecorded Latin adjective scūrus comes from the Proto-Indo-European root (s)keu-, (s)kū- (with variants) “to cover, envelop” ( scūrus therefore means “covered over”). In Germanic the variant skeu- forms the base of the noun skeujam “cloud cover, cloud,” becoming skȳ “cloud” in Old Norse, which is the immediate source of English sky (a 13th-century borrowing). The variant skū- forms the noun skūmaz “scum” (because it covers the water), which becomes scum in English.