Aristophanes,
Ecclesiazusae 372 (tr. Jeffrey Henderson):
Hey there, what are you doing? Not taking a shit, are you?
οὗτος τί ποιεῖς; οὔτι που χέζεις;
Suetonius,
Life of Vespasian 20 (tr. J.C. Rolfe):
He was well built, with strong, sturdy limbs, and the expression of one who was straining. Apropos of which a witty fellow, when Vespasian asked him to make a joke on him, replied rather cleverly: "I will, when you have finished relieving yourself."
statura fuit quadrata, compactis firmisque membris, vultu veluti nitentis: de quo quidam urbanorum non infacete, siquidem petenti, ut et in se aliquid diceret: "dicam," inquit, "cum ventrem exonerare desieris."
Martial 3.89 (my translation):
Use lettuce and use soft mallows:
for you have the look, Phoebus, of one who is taking a hard crap.
utere lactucis et mollibus utere malvis:
nam faciem durum, Phoebe, cacantis habes.