# What is the origin of "hydrogen-bonding"?

Hydrogen bonding is not due to $\text{anomalous expansion}$, whatever that is.
Hydrogen bonding is an intermolecular interaction observed when hydrogen is bound to a strongly electronegative atom such as fluorine, or oxygen, or nitrogen. It is largely responsible for the absurdly high normal boiling points of water, and hydrogen fluoride, $100$ ""^@C, and $19.5$ ""^@C respectively.
Because of the electronegativity of the heteroatom charge separation occurs to give ""^(delta+)H-F^(delta-) or ""^(delta-)OH_2^(delta+), and when the dipoles line up this constitutes a potent intermolecular force.