For what it's worth, this American speaker has never seen or used this word before.
undergird
[+ obj] chiefly US : to strengthen or support (something) from below — usually used figuratively ▪ Their way of life is undergirded by religious faith.
What does British English speakers use in place of "undergird"?
For what it's worth, this American speaker has never seen or used this word before.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
What do British English speakers use in place of "undergird"?
Underpin.