Both Uses of

stifle
in
The Scarlet Letter

It was as if a window were thrown open, admitting a freer atmosphere into the close and stifled study, where his life was wasting itself away, amid lamp-light, or obstructed day-beams, and the musty fragrance, be it sensual or moral, that exhales from books.
p. 114.4 *

stifled = stuffy (lacking fresh air that makes breathing easier)

And ever, after such an outpouring, oh, what a relief have I witnessed in those sinful brethren! even as in one who at last draws free air, after a long stifling with his own polluted breath.
p. 122.3

stifling = (figuratively) making breathing difficult

Definitions:

(1) (stifle as in: stifling the urge)
to suppress (prevent something or decrease its development) -- often political freedom
(2) (stifle as in: the heat is stifling)
to make breathing difficult or impossible -- often from heat or humidity
(3) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

Much more rarely, to stifle is used in the context of anatomy to refer to a four-legged animal's equivalent of the human knee (the joint between the upper and lower leg).