1. (

• hum

) sing with closed lips; "She hummed a melody" ;

2. (

• hum ,
• buzz ,
• seethe

) be noisy with activity; "This office is buzzing with activity" ;

3. (

• hum ,
• thrum ,

) sound with a monotonous hum ;

4. (

• hum ,

) make a low continuous sound; "The refrigerator is humming" ;