аавын хүү = эр хүн :) This kinds of expressions are used a lot: аавын хүү & ээжийн охин. It brings somewhat warm and personal meaning to what's being said. For example: Дайнд олон аавын хүү үрэгдсэн. One сан also say "Дайнд олон эрс үрэгдсэн". But the former not only says that many young men were killed at war, but also implies that many fathers (and mothers) lost their sons.