I think a Keyboard Word™ is one where:
When the numbers indicating the column of the letter on the left-hand keypad are typed out on the right-hand keypad, each number is adjacent to the next one in the series.
For example:
1 = Q, A, Z
2 = W, S, X
3 = E, D, C
4 = R, F, V
5 = T, G, B
6 = Y, H, N
7 = U, J, M
8 = I, K
9 = O, L
0 = P
$PASSED -> 012233$$PASSED \rightarrow 012233$
All the numbers are adjacent to each other on the left-hand keypad.
$FAILED -> 418933$$FAILED \rightarrow 418933$
Numbers 1 and 8 are not adjacent on the left-hand keypad.
$TILL -> 5899$$TILL \rightarrow 5899$
All the numbers are adjacent.
$UNTIL -> 76589$$UNTIL \rightarrow 76589$
Numbers 7 and 6 are not adjacent.
$ONES -> 9632$$ONES \rightarrow 9632$
All the numbers are adjacent.
$TWOS -> 5292$$TWOS \rightarrow 5292$
Numbers 9 and 2 are not adjacent.
This holds true for all the Keyboard Words™ and does not work for all the Not Keyboard Words™ so I believe it is what defines a Keyboard Word™.