Their delusions; or, their childish follies (LXX; ἐμπαίγματα). As God sends on some men "strong delusion that they should believe a lie" (2 Thessalonians 2:11), so on others he sends a spirit of childish folly, which makes their conduct silly and headstrong.
Persons whose characters are of this stamp are especially liable to vain and groundless "fears." When I called, none did answer (comp. Isaiah 65:12, and see the comment on that passage).