I am unable, yonder beggar cries,
To stand, or move; if he say true, he lies.
English is a great punning language, as John Donne skillfully shows here. This is the type of poem where you laugh despite yourself. You don't really expect to laugh in a poem about a lame (invalid) beggar. Who laughs at a beggar? Still, if the beggar is telling the truth, he lies. Lies on the ground, that is. It's the sort of pun that takes you by surprise and which must be rather disorienting for non-native speakers.