Keen as mustard

From Fresh Dictionary

English

Idiom

(to be) keen as mustard

1. ((idiomatic{{#if:|, {{{2}}}{{#if:|, {{{3}}}{{#if:|, {{{4}}}{{#if:|, {{{5}}}{{#if:|, {{{6}}}{{#if:|, {{{7}}}{{#if:|, {{{8}}}{{#if:|, {{{9}}}}}}}}}}}}}}}}}}})) To be very keen, enthusiastic, eager.

   He was keen as mustard to begin the new job, until he learned what it really entailed.