The etymology of "in memoriam" is rooted in Latin, which means "in memory of".  It is a Latin phrase consisting of the preposition in ("in") and the accusative form of memoria ("memory"), which comes from the word memor meaning "mindful" or "remembering".  It also gives us English words like memory, memorial, commemorate, and mourn. 

According to the Oxford English Dictionary (OED), the phrase in memoriam was first used in English before 1654, in the writings of Independent minister William Strong.  While used earlier, the phrase gained significant popularity in the 19th century, in part due to Alfred, Lord Tennyson's famous 1850 poem, In Memoriam A.H.H., written to mourn his friend Arthur Henry Hallam. 

Examples of "in memoriam" used in a sentence: 
"They held a concert in memoriam of the fallen soldiers." 
"The statue was erected in memoriam of the local hero." 
"The park was built in memoriam to honor the brave soldiers who sacrificed their lives for our country." 
"This scholarship is established in memoriam of Dr. Sarah Thompson, a renowned scientist who made significant contributions to her field."  
"After the event, a special broadcast was aired in memoriam of the television personality who had passed away." 
"School was closed today in memoriam of the President and will be closed tomorrow too, for his funeral." 

Just the facts:

IN MEMORIAM Definition & Meaning - Merriam-Webster