The Jargon File (version 4.4.7, 29 Dec 2003):
write-only language
n.
A language with syntax (or semantics) sufficiently dense and bizarre that
any routine of significant size is automatically write-only code. A
sobriquet applied occasionally to C and often to APL, though INTERCAL and
TECO certainly deserve it more. See also Befunge.
The Free On-line Dictionary of Computing (30 December 2018):
write-only language
A language with syntax (or semantics) sufficiently dense and
bizarre that any routine of significant size is automatically
write-only code. A sobriquet applied occasionally to C and
often to APL, though INTERCAL and TECO certainly deserve
it more.