Original Italian:

La donna è mobile.
 Qual piuma al vento,
 muta d'accento e di pensiero.
 Sempre un amabile,
 leggiadro viso,
 in pianto o in riso, è menzognero.
 
Refrain
 La donna è mobil'.
 Qual piuma al vento,
 muta d'accento e di pensier!
 e di pensier'!
 e di pensier'!
 
È sempre misero
 chi a lei s'affida,
 chi le confida mal cauto il cuore!
 Pur mai non sentesi
 felice appieno
 chi su quel seno non liba amore!
 
Refrain
 La donna è mobil'
 Qual piuma al vento,
 muta d'accento e di pensier!
 e di pensier'!
 e di pensier'!

English translation:

Woman is flighty.
 Like a feather in the wind,
 she changes in voice and in thought,
 always a lovely,
 pretty face,
 in tears or in laughter, she's [bad word] 
 Refrain
 Woman is flighty.
 like a feather in the wind,
 she changes in voice and in thought,
 and in thought!
 and in thought!
 
Always miserable
 is he who [bad word] her,
 he who confides in her his unwary heart!
 Yet one never feels
 fully happy
 who from that bosom does not drink love!
 
Refrain
 Woman is flighty.
 Like a feather in the wind,
 she changes her words,
 and her thoughts!
 and her thoughts!

Alternative translation:

woman is fickle (or erratic/flighty).
 Like a feather in the wind,
 she changes her words (You can't tell what she's really saying or thinking)
 and lovely face,
 graceful visage, (or beautiful face)
 that, in tears or in laughter, is deceitful.
 
Refrain
 (A) Woman is fickle.
 Like a feather in the wind,
 she changes her words,
 and her thoughts!
 and her thoughts!

Weeping or laughing, (or wretched)
 is [bad word] 
 He who confides in her his incautious (reckless) heart!
 Yet one never feels
 fully happy,
 who from that bosom/breast does not sip/taste love.

Refrain
 (A) woman is fickle.
 Like a feather in the wind,
 she changes in tone and in thought,
 and in thought!
 and in thought!