very easy, look at the word INFANT....A NEW BORN
BABY...SO SOMEONE behaving like an infant...is infantile.
Be careful not to confuse with "infantry" - (ground troops, soldiers who fight on foot)
We are looking for Content Writers (1-2 years experience) for our ed-tech startup based out of Gurgaon. If interested, please reach out to us at career@opencubicles.com