Old and Early Middle English did not yet have modal sentential adverbs of low probability. Old Norse did not have such words, either. From the 13th century onwards first epistemic prepositional phrases of Anglo-Norman origin functioning as modal adverbials consisting of the preposition per/par and nouns such as adventure, case, chance were borrowed into Middle English. In the late 15th century an analogous hybrid form per-hap(s), the combination of the Old French preposition per/par ‘by, through’ and the Old Norse noun hap(p) ‘chance’, both singular and plural, was coined according to the same pattern and was gradually grammaticalized as a univerbated modal sentence adverb in Early Modern English. The Norse root happ- was the source of some other new (Late) Middle English words which had no cognate equivalents in the source language: the adjective happy with its derivatives happily, happiness, etc. and the verb happen.
Together with another new Late Middle English formation may-be, a calque of French peutêtre, perhaps superseded the competing forms mayhap, (modal) happily, percase, peradventure, perchance, prepositional phrases with the noun hap and, finally, per-hap itself in Early Modern English after two centuries of lexical layering or multiple synonymy. The history of perhaps is a clear example of grammaticalization, whereby a prepositional phrase became a modal adverb now also used as a discourse marker. We find here all the typical features of the process: phonetic attrition, decategorization, univerbation, and obligatorification.
