Fromunder- +‎bar.

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underbar (pluralunderbars)

1. Abar placedunder asymbol.

   ${\displaystyle \underset{\_}{A}}$  is the letterA with anunderbar.

underbar (third-person singular simple presentunderbars,present participleunderbarring,simple past and past participleunderbarred)

1. (transitive) To place a bar under (a symbol).

• errabund,unbarred

Calque ofGermanwunderbar, equivalent tounder +‎-bar. CompareDanish andNorwegian Bokmålunderbar,Dutchwonderbaar,Germanwunderbar andLow Germanwunnerbor.

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underbar

1. wonderful,marvelous

• beundrar