From the definition found at Merriam-Webster and elsewhere, it seems that to ravel has completely opposite meanings; i.e. it means to unravel, to disentangle as well as to entangle.
What's going on here?
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Etymonline to the rescue:
1580s, "to untangle, unwind," also "to become tangled or confused," from Du. ravelen "to tangle, fray, unweave," from rafel "frayed thread." The seemingly contradictory senses of this word (ravel and unravel are both synonyms and antonyms) are reconciled by its roots in weaving and sewing: as threads become unwoven, they get tangled.
See also this related question: English words that are their own antonyms.