infantile

Infantile means immature or childish. Thumb-sucking and temper tantrums are examples of infantile behavior.

Doctors use the word infantile just like you'd expect: to describe things having to do with infants or babies. The rest of us, though, generally use this adjective when someone's being a brat or a crybaby or otherwise behaving immaturely. If someone refers to your sense of humor or your style of argument as "infantile" they are not offering you a complement. They might be trying to tell you to "grow up."

Definitions of infantile
1

adj of or relating to infants or infancy

infantile paralysis”

adj being or befitting or characteristic of an infant

infantile games”
Synonyms:
immature, young
(used of living things especially persons) in an early period of life or development or growth

adj indicating a lack of maturity

infantile behavior”
Synonyms:
childish
immature
characteristic of a lack of maturity