sigelo

The false seam first, because it is a tidy one. Sigelo looks like sig- plus a suffix -el-, and Esperanto has no suffix -el- at all. The affix a learner is half-remembering is -il-, the one that names a tool, and it genuinely applies here: sigelilo is the signet, the engraved thing you press with, while sigelo is the mark it leaves. That is a real derivation stacked on top of a root that is not itself derived, which is a common shape and worth recognising.

Three senses, all of them in use. The literal one is the impression pressed into wax or lead on a document or an envelope, to prove who sent it and that nobody opened it. From there the word goes figurative in two directions: a guarantee that settles something, as when a contract seals a friendship, and a closure that keeps something shut, as in sub sigelo de sekreto, under seal of secrecy. The verb is sigeli, and it carries all three.

The English trap. Seal is two separate words in English that happen to be spelled alike, the wax one from Latin and the animal one from Old English. Esperanto keeps them apart, as most languages do: the animal is foko. Writing la sigelo naĝis produces a swimming wax blob.

Related and not to be confused: signo is a sign or mark of any kind, stampo is a rubber stamp, poŝtmarko is a postage stamp, and subskribi is to sign your name, which is the modern replacement for the whole apparatus.