Gravitas:

• description- acting serious and formal.
• part of speech- noun
• related words- dignified(s), formal(s), careless(a)
• sentence- The gravitas at the symphony was too much for the kids to handle.

Alacrity:

• description- being eager to do something.
• part of speech- Noun
• related words- promptitude(s), fervor(s), dullness(a)
• sentence- The alacrity of the the girl impressed her boss on the first day of work.

Recalcitrant:

• description- rebelling against authority or being stubborn
• part of speech- adjective
• related words- rebellious(s), defiant(s), agreeable(a)
• sentence- The recalcitrant man refused to ask for directions.

Propensity:

• description- a natural tendancy toward something
• part of speech- noun
• related words- inclination(s), leaning(s), repulsion(a)
• sentence- The propensity of the pencil tapping on the desk was very annoying during the test.

Fallacious:

• description- tending to mislead or deceive.
• part of speech- adjective
• related words- beguiling(s), deceptive(s), true(a)
• sentence- The criminal tried to give a fallacious testimony.