Head over heels 本身是指“颠倒，完全地”，用在爱情上真是再合适不过了。To fall head over heels in love这个片语常指很快落入情海：
He fell head over heels in love with her. （= He fell for her head over heels = He is head over-heels in love with her.）（他与她沉溺情海，爱得晕头转向。）
