Sentence:  My lord, / Now fear is from me, I'll speak troth. Lord Cloten, / Upon my lady's missing, came to me / With his sword drawn; <WSD>foam'd at the mouth</WSD>, and swore, / If I discover'd not which way she was gone, / It was my instant death. 
Meaning of the word marked with <WSD> tags:  To rage, vent one's anger.