es faustdick hinter den Ohren haben

Etymology
lit. 'to have it fist-thick behind the ears'. The idiom is rooted in the old popular belief that waggishness and trickiness were little demons that sat behind people’s ears and were recognizable by thick bumps or lumps. Compare the German idiomatic expressions, ,.

Verb

 * 1)  to be a crafty, sly devil/fox, to be a sly dog, to be sly as a fox be crafty, sly, or occasionally impish and punchy in spite of one’s harmless, innocent, naive appearance
 * 2)  not to be as harmless, innocent, naive as it may seem at first glance; not to be underestimated
 * 1)  not to be as harmless, innocent, naive as it may seem at first glance; not to be underestimated
 * 1)  not to be as harmless, innocent, naive as it may seem at first glance; not to be underestimated
 * 1)  not to be as harmless, innocent, naive as it may seem at first glance; not to be underestimated
 * 1)  not to be as harmless, innocent, naive as it may seem at first glance; not to be underestimated