Definition from Wiktionary, the free dictionary
a- + semejar, or possibly from a Vulgar Latin *assimiliāre, from Late Latin assimilāre, present active infinitive of assimilō, from Latin ad + similō < similis. Compare Portuguese assemelhar, Italian assomigliare. Doublet of asimilar.
1 Argentine and Uruguayan voseo prefers the tú form for the present subjunctive.