1) Being one of 40 equal parts 2) Birthday many dread 3) Dreaded birthday for many 4) Next after the thirty-ninth 5) Ordinal 6) Ruby anniversary 
• (a.) Constituting one of forty equal parts into which anything is divided. • (a.) Following the thirty-ninth, or preceded by thirty-nine units, things, or parts. • (n.) One of forty equal parts into which one whole is divided; the quotient of a unit divided by forty; one next in order after the thirty-ninth.Fortieth: words in the d...
For'ti·eth adjective [ Anglo-Saxon feówertigo...a . See Forty .] 1. Following the thirty-ninth, or preceded by thirty-nine units, things, or parts. 2. Constituting one of forty equal parts into which anything is divided. For'ti·eth noun One of forty equal parts into which one whole is divided; the quotient of a unit divided by forty; one next in order after the thirty-ninth. 
[adj] - the ordinal number of forty in counting order 2. [n] - position 40 in a countable series of things
40th adjective the ordinal number of forty in counting order
next after the thirty-ninth; being the ordinal number for 40. · being one of 40 equal parts.