Determining the Meaning of Unknown Words

Pete was envious of his brother's new bicycle since he didn't get one.

Unlike my frugal mother, I don't shop bother shopping at sales or clipping coupons. 

Jeffery decided to sever his friendship with his rude neighbor.

Teenagers tend to be belligerent when arguing with their parents.

It has been an extremely brutal summer with very hot temperatures.

She offered a feeble excuse for her late work and they could tell she wasn't telling the truth.

The fifty pound pumpkin was gargantuan in size.

The sentence reads: "The new student was timid at first, speaking softly and avoiding eye contact." What does the word timid most likely mean?

The sentence reads: "The dark, ominous clouds gathered quickly, signaling that a storm was imminent." What does the word ominous most likely mean?

The sentence reads: "The athlete's stamina during the marathon amazed everyone, as she maintained a steady pace without tiring." What does the word stamina most likely mean?