Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
weak [wiːk] adj сла́бый*слаб (morally) слабохара́ктерный*слабохара́ктерен
to grow weak ослабева́ть or слабе́ть (ослабе́ть perf)
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.