1375–1425; late Middle English equivoc (< Late Latin aequivocus ambiguous, equivalent to Latin aequi- equi- + vōc-, stem of vōx voice + -us adj. suffix) + -al¹

1. See ambiguous.

The four-syllable word equivocal is sometimes said by those not entirely familiar with it as [ih-kwiv-uh-kuh-buh l] /ɪˈkwɪv ə kə bəl/, as if it were a five-syllable word, equivocable. This is probably the result of conflation with the pronunciations heard for many common adjectives that do end with -cable, as applicable, communicable, despicable, and eradicable.
However, if you split equivocal in half, as equi- + -vocal, the relation of its spelling to its origin and meanings becomes more clear. Think “equal voices,” two or more voices in conflict over a meaning, attitude, statement, etc., resulting in ambiguity, indecision, or deception. Recombine equi- + -vocal, put the main stress on the second syllable, and you have it: [ih-kwiv-uh-kuh l] /ɪˈkwɪv ə kəl/.
The form with the extra syllable is not found in educated writing, nor are any of its matching derivatives, like equivocably, unequivocable, and unequivocably. These are not considered standard variants and are best avoided in writing and speech.