In modern English usage, the word "fretful" is most commonly used to describe someone who worries a lot; who spends long hours chewing over a problem and is unable to push the worrying thoughts to the back of their mind for any length of time.
The word is not usually used to mean irritable or complaining these days, though it did have that alternative meaning in Shakespeare's time.
The word fretful is a synonym of the word antsy. It is usually used to describe a person who is unable to relax or sit still. A person who constantly whines or is habitually complaining is known as a fretful person.
Other synonyms of the word fretful are irritable, querulous, whiney, whiny, whining, complaining, complaintive, plaintive, worried, anxious, fussy, agitated, nervous and neurotic. The antonym of the word fretful is calm. A fretful person is one who is inclined or likely to be vexed, troubled or bothered. The word peevish is also a word which is similar in meaning to the word fretful.
A person who is characterised by worry and distress or a troublesome and restless person is usually referred to as a fretful person. Sir John, you are so fretful you cannot live long is a line from the play King Henry IV, Part 1 by William Shakespeare.