Someone who's insincere is a little bit fake or phony. Your classmate's insincere apology for interrupting you probably sounds like she doesn't really mean it.

When you're insincere, you're not being completely honest. A losing politician often gives an insincere-sounding concession speech, and a teacher who's having a terrible day might wear an insincere smile on her face. The adjective sincere, "coming from genuine, honest feelings," is at the root of insincere — the Latin prefix in, or "not," is combined with sincerus, "sound, pure, or whole."

Definitions of insincere

adj lacking sincerity

“a charming but thoroughly insincere woman”
“their praise was extravagant and insincere
counterfeit, imitative
not genuine; imitating something superior
dishonest, dishonorable
deceptive or fraudulent; disposed to cheat or defraud or deceive
artful, disingenuous
not straightforward or candid; giving a false appearance of frankness
lacking in reality or substance or genuineness; not corresponding to acknowledged facts or criteria
bootlicking, fawning, obsequious, sycophantic, toadyish
attempting to win favor from influential people by flattery
buttery, fulsome, oily, oleaginous, smarmy, soapy, unctuous
unpleasantly and excessively suave or ingratiating in manner or speech
concealing under a false appearance with the intent to deceive
deliberately deceptive
not genuine
gilded, meretricious, specious
based on pretense; deceptively pleasing
professing feelings or virtues one does not have
given to or characterized by presenting specious arguments
not in accordance with the fact or reality or actuality
open and genuine; not deceitful
echt, genuine
not fake or counterfeit
honest, honorable
not disposed to cheat or defraud; not deceptive or fraudulent
artless, ingenuous
characterized by an inability to mask your feelings; not devious
existent, real
being or occurring in fact or actuality; having verified existence; not illusory
bona fide
undertaken in good faith
sincerely or intensely felt
dear, devout, earnest, heartfelt
without dissimulation; frank
genuine, true, unfeigned
not pretended; sincerely felt or expressed
heart-whole, whole-souled, wholehearted
with unconditional and enthusiastic devotion
consistent with fact or reality; not false
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