The major Old English adjective of certainty was (ge)wiss, which in early Middle English came to be replaced with sicker derived from very weakly attested Old English sicor, a word of ultimate Romance origin (from Latin sēcūrus). The relative paucity of occurrences of both adjectives in the Dictionary of Old English corpus is attributed to their use in mostly spoken language. The rapid increase in the usage of sicker in the thirteenth century is a mystery with possible, yet difficult to prove, Norse and/or Anglo-Norman influence. The fourteenth century marks the appearance of sure and certain borrowed from Anglo-Norman first by bilingual speakers and writers, and the quick diffusion of the new lexemes to all dialects and genres. This article looks at the adoption of the different senses of these polysemous adjectives into Middle English in the context of subjectification, which appears to affect not only semantic developments within one language but also the process of borrowing. When sure and certain were used epistemically, they tended to occur in the predicative position, usually following the copula. It took several centuries of lexical layering (coexistence of synonyms) before sicker was lost from Standard English in the sixteenth century.