knowledge

First recorded in 1250–1300; Middle English knouleche, equivalent to know(en) “to know” + -leche, perhaps akin to Old English -lāc suffix denoting action or practice, cognate with Old Norse (-)leikr; see know¹; cf. wedlock

1. See information.

Know, the first half of knowledge, is a no-brainer, so to speak. Know comes from the Proto-Indo-European root gnō-, gnē- gen-, gṇ- “to know, recognize.” The variant gnō- appears in Latin (g)nōscere “to know, come to know” (the -sc- is an inchoative or inceptive infix indicating the beginning of an action). Greek gignṓskein shows the same variant gnō- as Latin, preceded by present-tense reduplication, which in Greek is the first consonant of the root followed by i; gnō- is followed by the same inceptive infix -sk-. The Germanic forms of the root are knā-, knē-, kun-. The variant knē- forms the verb knēwan, Old English cnāwan, English know. The variant kun- forms the Old English verb cunnan “to be or become acquainted with, to know” (the English auxiliary verb can ) as well as the adjective and noun cunning.
The real problem is the second element, -ledge. There are many, many Middle English spelling variants of knowledge, including knoulecch(e), knouelech(e), cnoulech, knowlesche, knoleche, and later spellings, including knoulegge, knoleg(e), knoleige, knowlegege, knaulag(e), cnaulage. The earlier spellings indicate a pronunciation with the same ch sound as in leech or letch; the later spellings, for example, knaulage, originating in the northern dialects and implying a pronunciation with a soft g (as in gem ), appear in the 15th century.
The Middle English element -lech(e) is a very rare noun suffix from unrecorded Old English -lǽce, a variant of -lāc, a noun suffix indicating action or proceeding. The original suffix survives only in the noun wedlock (Old English wedlāc “pledge, security, espousals,” Middle English wedlōk “institution of marriage, the married state”), proving, once and for all, that wedlock is not related to the word lock.