Antonyms of adj on_one's_guard

1 sense of on one's guard

Sense 1
on guard, on one's guard (predicate), upon one's guard (predicate), on your guard -- (vigilant; "on guard against con artists"; "must remain on your guard in such a situation")

INDIRECT (VIA wary) -> unwary -- (not alert to danger or deception; "the shrieks of unwary animals taken by surprise"; "some thieves prey especially on unwary travelers"; "seduce the unwary reader into easy acquiescence"- O.J.Campbell)

Similarity of adj on_one's_guard

1 sense of on one's guard

Sense 1
on guard, on one's guard (predicate), upon one's guard (predicate), on your guard -- (vigilant; "on guard against con artists"; "must remain on your guard in such a situation")
       => wary (vs. unwary) -- (marked by keen caution and watchful prudence; "they were wary in their movements"; "a wary glance at the black clouds"; "taught to be wary of strangers")