by chance

Từ điển WordNet v3.1 - WordNet Dictionary

adverb

1. through chance, "To sleep, perchance to dream.."
• Syn:
perchance
• Usage Domain:
archaism, archaicism
2. by accident
- betrayed by a word haply overheard
• Syn:
haply, by luck
3. without advance planning
- they met accidentally
• Syn:
accidentally, circumstantially, unexpectedly
• Ant:
deliberately (for: accidentally)
• Derived from adjective:
unexpected (for: unexpectedly)