There are two Mays — Compromiser's Rebellion

The key word is compromise in line four. People tell the rebellious young what they may do, must do, shall do, and they just do what they will. Dickinson compromised with the world by rebellious riddles.

There are two Mays ¹
And then a Must
And after that a Shall.
How infinite the compromise
That indicates I will!

(F.1637/J.1618)

[1] two Mays:: "You may do this; you may do that."
[2] a Must:: "You must do this."
[3] a Shall:: "You shall do that."
[4] compromise:: the involvement in danger or risk. infinite:: Dickinson compromised with the world and wrote her will in riddles with infinite effort.
[5] I will:: "I will do what I will."