Assess comes from Middle English assessen, from Old French assesser, from Late Latin assessāre “to fix a tax on.” Assessāre is a Late Latin frequentative verb derived from assess-, the inflectional stem of the past participle assessus, from the Latin verb assidēre “to sit next to or by (as an assistant, attendant, or aide),” formed from the preposition and prefix ad, ad-, here having the sense “nearness, presence at,” and -sidēre, a combining form of the verb sedēre “to sit, be seated.”
In Proto-Indo-European, two dental consonants (such as d + d, d + t, t + t, etc.) could not appear together. In the Italic languages (Latin, Oscan, Umbrian) and Germanic, the two dental consonants developed into -ss- ; thus the original Latin past participle of sedēre , sedtus (originally an adjective suffix, typically forming past participles in Latin) regularly became sessus, the base for the Late Latin verb assessāre.

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