Off one's rocker

From Fresh Dictionary

English

Adjective

off one's rocker

1. ((idiomatic{{#if:|, {{{2}}}{{#if:|, {{{3}}}{{#if:|, {{{4}}}{{#if:|, {{{5}}}{{#if:|, {{{6}}}{{#if:|, {{{7}}}{{#if:|, {{{8}}}{{#if:|, {{{9}}}}}}}}}}}}}}}}}}})) crazy; insane

   He's off his rocker if he thinks he can tackle the whole thing alone.