1839, "gland-like," from medical Latin adenoideus, from Greek adenoeides, from adēn (genitive adēnos) "gland" (see adeno-) + eidos "form" (see -oid). Adenoids (n.) "adenoid growths" is attested by 1856.
scientific word-forming element meaning "gland," from Greek adēn "gland," which is perhaps from a suffixed form of PIE root *engw- "groin; internal organ" (source also of Latin inguen "groin"), but Beekes rejects all cognates and calls it isolated.
word-forming element meaning "like, like that of, thing like a ______," from Latinized form of Greek -oeidēs (three syllables), from eidos "form," related to idein "to see," eidenai "to know;" literally "to see" (from PIE *weid-es-, from root *weid- "to see"). The -o- is connective or a stem vowel from the previous element. Often implying an incomplete or imperfect resemblance to the thing indicated.