Definition of "obligatory"

[…] if he speak the words of an oath in a strange language, thinking they signify something else, or if he spake in his sleep, or deliration, or distraction, it is no oath, and so not obligatory.

1673, Richard Baxter, Christian Directory

[I]t was something that every schoolboy of my generation almost `had' to do, as obligatory a proof of impending manliness as scrumping apples or pulling girls' pigtails.

2000, Bill Oddie, Gripping Yarns, page 12