Wordnet 3.0
VERB (1)
1. 
 fall or descend to a lower place or level; 
- Example: "He sank to his knees"[syn: sink, drop, drop down]
WordNet (r) 3.0 (2006):
drop down
    v 1: fall or descend to a lower place or level; "He sank to his
         knees" [syn: sink, drop, drop down]