I am most likely to lookup the definition of a word when:

• I am in the mood
• I am proofreading
• The usage seems wrong
• I don't know what it means
• I am not sure what it means
• I'm not positive what it means
• I am afraid i used it incorrectly
• Someone challenges my use of it
• Context might change its meaning
• I want to see how it is pronounced
• It confuses what the sentence means
• I happen to have a dictionary nearby
• I want to know the word's etymology
• The dictionary plugin/app is working
• I want to see it contrasted to other words
• Never. I can (almost) always tell from context
• If i don't know what it means, it must not be important