Of or pertaining to Proteus; characteristic of Proteus.

Exceedingly variable; readily assuming different shapes or forms; as, an amœba is a protean animalcule.

Proteus was a sea god who could change shape and who possessed knowledge of the future. To obtain information, one had to grapple with him until his metamorphoses ceased. Protean means of changeable or variable form, or having the ability to change form.

taking on different forms; "eyes...of that baffling protean gray which is never twice the same"