adj. 25B6; adjective
her treacherous brother betrayed her : TRAITOROUS, disloyal, faithless, unfaithful, duplicitous, false-hearted, deceitful, false, back-stabbing, double-crossing, double-dealing, two-faced, untrustworthy, unreliable; apostate, renegade; informal two-timing; poetic/literary perfidious.
treacherous driving conditions : DANGEROUS, hazardous, perilous, unsafe, precarious, risky, deceptive, unreliable; informal dicey, hairy.
loyal, faithful, reliable.