etymologeek.com dictionary
English word weaken comes from English -en, English weak.

Weaken etymology history

Weaken detailed word origin explanation

Dictionary entryLanguageDefinition
-en English (eng) (dialectal, or, non-standard, rare) Used to form the plural of nouns. Can be used to denote the plural form of a small number of English words, the majority of whose etymology goes back to the n-stem (i.e. weak noun) declension of Germanic languages. Used to form the diminutives of certain nouns. When attached to certain adjectives, it forms a transitive verb whose meaning is, to make [...]
weak English (eng) (Germanic languages, of adjectives) Definite in meaning, often used with a definite article or similar word.. (Germanic languages, of nouns) Showing less distinct grammatical endings.. (Germanic languages, of verbs) Regular in inflection, lacking vowel changes and having a past tense with -d- or -t-.. (chemistry) That does not ionize completely into anions and cations in a solution.. [...]
weaken English (eng) (intransitive) To become weaker.. (transitive) To make weaker.

Words with the same origin as weaken

Descendants of -en
awaken beaten brethren brighten chicken freshen golden lighten loosen straighten strengthen tighten wooden