Etymology 1 
From Middle English fikil, fikil, from Old English ficol (“fickle, cunning, tricky , deceitful”), equivalent to fike + -le. More at fike.
fickle (comparative fickler or more fickle, superlative ficklest or most fickle)
- Quick to change one’s opinion or allegiance; insincere; not loyal or reliable.
Derived terms 
quick to change one’s opinion or allegiance
Etymology 2 
From Middle English fikelen, from fikel (“fickle”); see above. Cognate with Low German fikkelen (“to deceive, flatter”), German ficklen, ficheln (“to deceive, flatter”).
fickle (third-person singular simple present fickles, present participle fickling, simple past and past participle fickled)
- (transitive) To deceive; flatter.
- (transitive, UK dialectal) To puzzle; perplex; nonplus.