describes that someone has a tendency to (... do some action): It denotes repetetive, negative, tendency. it comes from 〜がる, but nominalised.
なんというか、人を見下みおろしがちなタイプだったんですかね。 How should I put it? He was the type that would often look down on people.
{部活|ぶかつ}休みがちだったけど… She tended to miss a lot of club activities though...
ヨナは病気がちであまり外に出れないから。 Yonah is very prone to falling sick, so therefore she doesn't go out very often.