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

adj of or relating to infants or infancy

infantile paralysis”

adj being or befitting or characteristic of an infant

infantile games”
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”
characteristic of a lack of maturity