Frailty, thy name is woman
Alluding to the alleged inherent weakness of character of women.
From Shakespeare's Hamlet, 1602:
Must I remember? why, she would hang on him,
As if increase of appetite had grown
By what it fed on: and yet, within a month -
Let me not think on't - Frailty, thy name is woman! -
A little month, or ere those shoes were old
With which she follow'd my poor father's body...
Hamlet is angry that his mother, Gertrude, has married his uncle Claudius within a month of his father's death. The speech generalizes the attribution of weakness of character from one particular woman to womankind.
See other phrases and sayings from Shakespeare.