What does hamesameach mean in Hebrew?

Answer 1

Samekh or Simketh is the fifteenth letter of many Semitic abjads, including:

Phoenician Ṣāmek ?

Hebrew ˈSamekh ס

Ugaritic Samka ?

Aramaic Semkath ?

Syriac Semkaṯ ܣ, ܤ representing /s/

The Arabic alphabet is the only Semitic alphabet that doesn't have a samekh. Instead, it uses a letter based on Phoenician Šīn to represent /s/; however, that glyph takes Samekh's place in the traditional Abjadi order of the Arabic alphabet.

The Phoenician letter gave rise to the Greek Xi (Ξ, ξ)

The origin of Samekh is unclear. The Phoenician letter may continue a glyph from the Middle Bronze Age alphabets, either based on a hieroglyph for a tent peg / some kind of prop (s'mikhah, Hebrew: סמיכה‎‎, or t'mikhah, Hebrew: תמיכה‎‎, in modern Hebrew means to support), and thus may be derived from the Egyptian hieroglyph djed.

Answer 2

ha mesameach (המשמח) = "the happiness causer" or "the gladdener".