Donald Trump said about Adam Schiff that "he could not carry his 'blank' strap". Later, in the tonight show starring Jimmy Fallon Natalie Portman referred to this phrase, when Fallon said her a statement "insult a kid would say".
I've found that Trump couldn't say the word "jockstrap" but I still don't get this joke.
Why did he say it about Schiff, what did this phrase mean in that context and why did Natalie Portman say it in a given situation?
Is it some kind of a cultural joke? If it is, could you please explain it?
Thank you in advance