Both Uses of

sheer

in

Lord of the Flies

...and when one tried to get the thing straight the argument sheered off, bringing up fresh, unpleasant matter.
p. 88.9 *

sheered = abruptly changed direction

So they proceeded after that as if they were climbing a dangerous mountain, until the rocks became an uncompromising cliff, overhung with impossible jungle and falling sheer into the sea.
p. 117.1 *

sheer = so steeply as to be almost vertical

Definitions:

(1) (sheer as in: a sheer blouse)
typically of fabric: very thin and delicate -- often transparent
(2) (sheer as in: sheer fun)
complete or pure -- without restriction, qualification, or other elements (used for emphasis)
(3) (sheer as in: a sheer cliff)
so steep it is almost vertical
(4) (sheer as in: sheer to the left)
change direction; or to cause such a change of direction -- usually abruptly
(5) (meaning too rare to warrant focus)
Sheer can also be an alternative spelling for shear (to cut).