Several phrases have double meanings:

    “mad north-northwest”

– A compass… bit off. Hamlet is only a little bit off his rocker.

– North-northwest is just one of sixteen points on the compass. Hamlet is only crazy once in a while. 

“When the wind is southerly”

– In England … clear skies (and presumably clear minds).

– This wind clears away the smokescreen put up by Claudius, allowing Hamlet to see what’s going on.

    “I know a hawk from a hand saw (or heronshaw – the two words sounded almost identical)”

– A hand saw is very different from a hawk. Even in his distrubed emotional state, Hamlet can see the obvious, that Rosencrantz and Guildenstern are agents of the king and queen.

–A heronshaw is another species of bird. Hamlet’s only pretending to be crazy; he can distinguish between bird species, as well as see through the duplicitous nature of Rosencrantz and Guildenstern.