This is the 950th most frequent Vietnamese word.
đứa
"Đứa" means "child" or "person" and is often used to refer to someone in an informal or familiar way.
In this sentence, 'đứa' is used as a classifier for 'trẻ', indicating a child or young person.
Đứa trẻ đó rất thông minh.
That child is very intelligent.
In this case, 'đứa' is used metaphorically to refer to an individual (oneself) in a somewhat humble or self-reflective tone.
Tôi không phải là đứa hay quên.
I am not someone who forgets often.
Here, 'đứa' refers generally to children and is used in the plural form 'các đứa'.