полезный

Etymology
From. The form of this adjective is the regular outcome of the Proto-Slavic stem ; the first yer was strong due to the presence of another yer in the following syllable. The actual genitive plural of  is phonologically unexpected, and was formed by analogy.

Adjective

 * 1) useful, helpful, good, beneficial having a practical or beneficial use
 * 2) wholesome
 * 3) usable