I have seen it from a sentence in an eye drop instruction manual. Here it is.
かつてない気持ちいい、さし心地...それは、極小一滴だから！
In my opinion, I translate it as "the very feeling" by referring 「さし」from a source that it is a prefix used to emphasize the following word, in this case is 「心地」. Therefore, the whole word 「さし心地」 would function as stressing the previous one 「気持ちいい」to inform that it will really help making the user feel great after using it.
"to apply eye drops" = 「[目薬]{めぐすり}を[差]{さ}す」
Thus, 「さし」 in 「さし[心地]{ごこち}」 is the [連用形]{れんようけい} (continuative form) of the verb 「差す」.
(Note that 心地, all by itself, is read ここち, but with a 連用形 in front, it is read ごこち.)
「Verb in [連用形]{れんようけい} (continuative form) + [心地]{ごこち}」
= "degree or level of pleasantness/comfortableness in performing the action describing by the verb"
So, 「（[気持]{きも}ちいい）さし心地」 refers to how pleasantly one can apply this particular eye drop.
The form 「～～心地」 is used quite often and here are some examples.
[住]{す}み心地: How comfortable to live in (said of a home or town/area).
[聴]{き}き心地: How pleasant to listen to (said of music or language).
[寝]{ね}心地: How comfortable to sleep in (said of futon or bed).
I think 心地 means nearly "feeling" and ～心地 means "the feeling of doing something"
For example, 着心地(the feeling of wearing it), 触り心地(the feeling of touching it), 履き心地(the feeling which you get on it).
To put eyedrops in one's eye is translated as 目薬をさす in Japanese, so さし心地 means "the feeling of putting eyedrops in your eye".
