understanding a passage with "tenir" as well as "en prenant"

Question

(Warning: This question contains somewhat sexually explicit material).

The following passage confuses me a lot:

Je lui ai tenu fermement les hanches en prenant mon pied et lui aussi. Il n'est habituellement pas très fan de cette position, mais là il prenait vraiment son pied.

1) Understanding "tenir" with both an indirect object and a direct object.

"Je lui ai tenu fermement les hanches".

I go to wordreference's page for "tenir". To understand what it means to "tenir" "les hanches", I look at all the entries that are marked "vtr" (transitive verbs), which, if you mouse over the "vtr" symbol, wordreference indicates that a "vtr" takes a direct object. I conclude that to "tenir" the hips is to grab hold or take hold the hips.

Then, to understand "lui", I look at all the entries that have "à qqn". I conclude that it could mean "to [physically] cling or stick onto someone", or "to mean a lot to someone". Given that this passage is a physical description of what's happening during sex, I'm guessing that it's the first.

So, so far I have "I held firmly to his hips and clung onto him". So far, so good. But then I question whether this is a correct translation, as I translate the rest of the sentence:

2)

.. en prenant mon pied et lui aussi.

".. while holding my foot and him too".

How does a person both hold firmly onto someone's hips, while also holding onto your own foot [?!] ? And why does the narrator even mention "en prenant [..] lui" in addition to saying "je lui ai tenu" -- isn't this redundant, given they pretty much mean the same thing ("while holding onto him" and "I clung onto him")?

3) Strangely, the next sentence also mentions holding onto a foot:

.. il prenait vraiment son pied.

"he was really holding onto his foot".

At this point I'm suspecting that "prendre son pied" is an expression, but neither linguee.fr nor wordreference seems to confirm this.

Questions:
1. Is my guess of how to translate "Je lui ai tenu fermement les hanches" correct? Is my understanding correct that, when I look in a dictionary, that I need to look for entries in a dictionary that say "transitive verb" to understand the direct object, and to look at entries with "à [qqch/qqn]" to understand the indirect object?
2. Is it true that "Je lui ai tenu" and ".. en prenant [..] lui aussi" is redundant?
3. What does "prendre son pied" mean? Is there a way I could have looked this up, myself?

Answer 1

(1) Je lui ai tenu fermement les hanches

Your strategy for understanding tenir is a good start, but you shouldn't combine two definitions where one takes a direct object and the other an indirect object. Instead, if the verb can take both at once, you'll find an entry that shows both.

For example, here's a WordReference subheading for donner that accounts for each object in this complex-looking set simultaneously:

So why does your tenir have both a direct and an indirect object if there's no subheading tenir [qqch] à [qqn] ? Answer: lui identifies the owner of the hanches. This would be hard to interpret until you know this pattern: Body parts can be connected to their owner by an indirect object.

This might sound somewhat absurd, but you'll recognize it from familiar phrases like these:

Je me lave les mains.

Tu te brosses les dents.

Anglophones are tempted to say « Je lave mes mains » and « Tu brosses tes dents ».

So the phrase you quoted is like « J'ai tenu fermement ses hanches » (if you could say that).

Generally, French conceptualizes body parts oddly. It seems to avoid possessive + body part as often as possible. For example, one doesn't say « Ma tête tourne » but « J'ai la tête qui tourne ».

(2–3) il prenait vraiment son pied

You were right to suspect that this odd-sounding position is due to its being an idiom. I'm not sure why your search on WordReference failed to turn one up. Here's the entry:

have a blast, have a great time, have a ball

I switched the language to French-English and watched for new suggestions as I typed each letter.