ことだし(And) so, Because, Since

ことだし is a combination of the noun こと 'thing', the auxiliary verb だ, and the conjunction particle し, used for listing reasons. The whole structure itself often translates simply as 'because (A)', or 'since (A)', but places extra emphasis on there being other reasons besides what the speaker has specified.

This structure may appear after any word in its attributive form, or after a noun that has been followed with の.

• せっかく君(きみ)のお父(とう)さんとお母(かあ)さんが来(く)ることだし、どっかいい焼肉屋(やきにくや)さんにでも行(い)こう。

  Since your father and mother are coming all the way (to see us), let's go to a nice yakiniku restaurant.
• 天気(てんき)もいいことだし、みんなで公園(こうえん)にでも行(い)きましょう。

  Since the weather is nice, let's all go to the park together.
• 彼(かれ)が勤(つと)めている会社(かいしゃ)は有名(ゆうめい)なことだし、給料(きゅうりょう)もいいんだろう。

  Since the company he works for is famous, his salary must be good.
• 彼(かれ)のことだし、また遅(おく)れてくるんじゃないの。

  Knowing him, I bet that he will come late as usual.

Just like with its standard use in creating lists, し is being used in this grammar point to show that (A) is not exclusive. The way this comes across is that 'while there are other reasons, (A) is good enough to (B)'. (B) is usually some suggestion, invitation, or expression of determination.