This is the 15889th most frequent Russian word.
пропавшего
"Missing" or "lost."
In this sentence, 'пропавшего' (missing) is used as an adjectival participle modifying the noun 'человека' (person).
Мы ищем пропавшего человека.
We are looking for the missing person.
Here, 'пропавшего' functions as a noun, implying 'the missing one', without explicitly mentioning 'person'.
Найдены вещи пропавшего.
The belongings of the missing one have been found.
In this example, 'пропавшего' (lost) modifies the noun 'багажа' (luggage) describing its state.