festbeißen fest+beißen vr sep irreg
[Hund etc] to get a firm hold with its teeth ( an +dat on)
[Zecke etc] to attach itself firmly ( an +dat to) (fig)
(=nicht weiterkommen) to get bogged down ( an +dat in)
der Hund biss sich an ihrem Bein fest the dog sank its teeth firmly into her leg