“老” means “old,” and “阴阳师” (literal meaning: yin-yang master) refers to fengshui masters who tell fortunes and help people conduct ceremonies to pray for blessings or avert misfortunes. However, Chinese netizens use the term to refer to those who like to produce pertinent, sometimes mean and harsh comments on people or things. The usage could have derived from the phrase “阴阳怪气” (yīnyáng guàiqì; meaning “［speaking］in an odd irritating manner”).
Wǒ xīnmǎi de zhèjiàn yīfú hěn nánkàn ma？Lìlì shuō xiàng lǎowūpó de páozi。
Is this outfit I just bought really ugly? Li Li said it resembled a robe an old witch would wear.
Bié lǐ tā，tā shì gè lǎoyīnyángshī。
Just ignore her. She is always saying mean things about others.