Solidly means in a firm, strong, or reliable way. When a house is solidly built, its walls are sturdy and its foundation won't crack or shift. When you understand something solidly, you know it deeply and completely, with genuine mastery.

The word often describes physical strength or stability. A solidly constructed bridge can handle heavy trucks without swaying. A football player who's solidly built has a strong, muscular frame. But it also describes non-physical things: a student who performs solidly on tests consistently does well, and a friendship built solidly on trust and honesty won't fall apart over small disagreements.

You might also hear people say a team played solidly when they performed competently throughout the whole game, without major mistakes. Or that someone is solidly in favor of an idea when they support it firmly, without wavering.

The word suggests something you can count on. When something is done solidly, it's done thoroughly and dependably, the kind of work that lasts.