The Lord is good,
A stronghold in the day of trouble;
And He knows those who trust in Him.
Nahum 1:7, NKJV
The word "stronghold" is the hebrew word mauwz (maw-ooz'); or mahoz (maw-oze'). It means a fortified place, a defense, force, fort, fortess, rock, strength.
It comes from the root word `azaz (aw-zaz'), which means harden, impudent, prevail, strengthen, be strong.
*Strong, James, LL.D., S.T.D., The New Strong's Expanded Exhaustive Concordance of the Bible, Red Letter Edition (Expanded With the Best of Vine's Dictionary of Old & New Testament Words) (Nashville: Thomas Nelson, 2010).