light up

1.  Become or cause to become more animated or cheerful, as in Her laughter lit up the whole room, or His face lit up when he saw her. This expression transfers physical illumination to human moods. Also see lighten up. [Mid-1700s]
2.  Start smoking a cigar, cigarette, or pipe, as in The minute he got outside the church he lit up. [Colloquial; mid-1800s]

The verb has 5 meanings:

Meaning #1: start to burn with a bright flame

Meaning #2: make lighter or brighter
  Synonyms: light, illume, illumine, illuminate

Meaning #3: become clear
  Synonyms: clear up, clear, brighten

Meaning #4: ignite

Meaning #5: begin to smoke
  Synonyms: fire up, light