In Act 1
Ragueneau and Le Bret discuss Cyrano's nose. They are joking if the nose is real or not, and Le Bret responds "He keeps it on, - and cleaves in two any man who dares remark on it!"
"He keeps it on" simply means it really is his nose
and "cleaves in two" he will fight anyone who makes fun or "dares remark on it."
