1. *Perspicacious* (adjective): Having a keen understanding and insight.
Sentence: "She was a perspicacious observer of human behavior."
2. *Ennui* (noun): A feeling of listlessness and boredom.
Sentence: "After a few months of doing the same job, he started to feel ennui."
3. *Fastidious* (adjective): Meticulous and demanding in one's standards.
Sentence: "He was a fastidious editor, ensuring every detail was correct."
4. *Heterogeneous* (adjective): Composed of diverse or different elements.
Sentence: "The city's population was heterogeneous, with people from various cultures."
5. *Inscrutable* (adjective): Difficult to understand or interpret.
Sentence: "The ancient hieroglyphics were inscrutable to most scholars."
6. *Lassitude* (noun): A feeling of weariness or lack of energy.
Sentence: "After a long, hot summer, the team felt lassitude."
7. *Malleable* (adjective): Capable of being shaped or influenced.
Sentence: "Children's minds are malleable, making them receptive to learning."
8. *Nebulous* (adjective): Unclear or vague in meaning or form.
Sentence: "The concept of happiness can be nebulous and subjective."
9. *Obfuscate* (verb): To make something unclear or difficult to understand.
Sentence: "The politician's answer obfuscated the truth."
10. *Perfidious* (adjective): Disloyal or treacherous.
Sentence: "The company's perfidious actions led to a loss of customer trust."
11. *Precarious* (adjective): Not secure or stable.
Sentence: "The precarious balance of power made it difficult to negotiate."
12. *Reclusive* (adjective): Preferring to be alone or avoiding social contact.
Sentence: "The reclusive author rarely gave interviews."
13. *Sagacious* (adjective): Having keen discernment and good judgment.
Sentence: "The sagacious investor predicted the market trend."
14. *Taciturn* (adjective): Unwilling to talk or communicate.
Sentence: "The taciturn stranger kept to himself."
15. *Ubiquitous* (adjective): Present or found everywhere.
Sentence: "Smartphones are ubiquitous in modern life."
16. *Vacillate* (verb): To hesitate or be indecisive.
Sentence: "She vacillated between two job offers."
17. *Vigilant* (adjective): Watchful and attentive.
Sentence: "The vigilant security guard spotted the intruder."
18. *Whimsical* (adjective): Playful or fanciful.
Sentence: "The artist's whimsical style delighted audiences."
19. *Xenial* (adjective): Friendly and hospitable to guests.
Sentence: "The xenial host made us feel welcome."
20. *Yield* (verb): To give in or surrender.
Sentence: "The company will yield to the competitor's offer."