Silly
Definition 1 of 5
Pro Tip 1/2
Lack of seriousness
Use 'silly' to describe actions or decisions that are not taken seriously or done playfully. They wore silly hats to the party just for fun.
Pro Tip 2/2
Negative connotation
'Silly' can imply a mild reprimand or disappointment in someone's actions. Making a silly mistake like that could have been avoided.