The technical details were 'over my head'. [=too complicated for me to understand]
She started to ask a question and then her voice 'tailed off'. [='trailed off']
His handwriting was so bad that we 'couldn't make heads or tails out of' it.
(..) DEL: Please, John. Help him. Please, Johnny, help him. Oh, dear Jesus. The tail. Look at the tail. What did you do? I helped Del's mouse... ...he's a circus mouse. Gonna live in a mouse city... (..)1999 The Green Mile