O, the month of May, the merry month of May,
So frolic, so gay, and so green, so green, so green!
O, and then did I unto my true love say,
Sweet Peg, thou shalt be my Summer's Queen.
Now the nightingale, the pretty nightingale,
The sweetest singer in all the forest quire,
Entreats thee, sweet Peggy, to hear thy true love's tale:
Lo, yonder she sitteth, her breast against a brier.
But O, I spy the cuckoo, the cuckoo, the cuckoo;
See where she sitteth; come away, my joy:
Come away, I prithee, I do not like the cuckoo
Should sing where my Peggy and I kiss and toy.
An inside joke, now long obsolete. The cuckoo in Elizabethan times was associated with cuckoldry, i.e., wives cheating on husbands. Supposedly the cuckoo bird called whenever husbands were getting cuckolded. 
 
 Hey! Your wife’s fucking somebody else. Heh heh heh. Pretty CUCKOO, right? Ha ha ha! No, but seriously, she is.
The Merry Month of May