Perhaps after having read Macbeth and after having witnessed what kind of relationship Macbeth develops with the preternatural world, the reader may infer that the first witch implies that she will make Macbeth weak, drying him of his masculinity and strength of character which enables him to control his own actions.
While Macbeth in the first act does listen to the three witches as they foretell his becoming Thane of Cawdor, he remarks aside
This supernatural soliciting
Cannot be ill, cannot be good. (I,iii,130-131)
Still, he is intrigued and wonders, in another aside,
If chance will have me King, why, chance may crown me,
Without my stir. (I,iii,144-145)
Later, in Act II, Macbeth replies to Banquo's remarks about his having dreamt of the "three weird sisters": "I think not of them" (II,i,22). However, by the latter acts of the play, Macbeth seeks the witches to hear what they next predict. After killing Duncan and having Banquo killed, but missing in the murder of Banquo's son, Macbeth--true to the witch's words--is worn out with fatigue as he sees Banquo's ghost. Truly he does live "like a man forbid," worn out with anxiety about anyone who can take the throne from him. He feels compelled to visit the "weird sisters" again in Act IV. The first witch tells him to beware the thane of Fife, and the second tells him that he cannot be harmed by anyone of woman.
Haunted by these prophesies and under the spell of the witches whose one member says she will do harm the complete number of three times (always a significant and spiritual number), Macbeth has succumbed to the power of the preternatural world; he does, indeed, live like a man under a curse, worn out with fatigue as the first witch predicts,
He shall live a man forbid.
Weary sen'nights nine times nine
Shall he dwindle, peak, and pine;
Though his bark cannot be lost,
Yet it shall be tempest-tost.
Look what I have. (I,iii,21-25)